This invention relates to magnetic sensing devices for measuring the relative strength of a magnetic field or magnetic field gradient. More particularly, this invention relates to magnetic sensing devices in which the active elements are fabricated from superconducting materials and maintained at superconducting temperatures during use.
Superconducting magnetic sensing devices are known in which one or more sensing coils are employed to measure the relative strength of a magnetic field or a field gradient. In typical devices of this type, the sensing coils are fabricated from a superconducting material and are maintained during use at an extremely low temperature at which the material is rendered superconductive.
In order to produce reliable results with such devices, it has been found necessary to provide means for initially adjusting or balancing the effective areas of the sensing coils. In one such arrangement, each sensing coil is balanced by connecting in series with the coil an auxiliary trim coil oriented at an angle to the plane of the sensing coil. By adjusting the area enclosed by the trim coil and the angle of the trim coil relative to the plane of the sensing coil, the desired initial balance condition is attained. However, in order to balance the device, the sensing and trim coils must first be removed from the superconducting environment, after which the trim coils may be adjusted to an empirical area and angular orientation, and the newly-configured coils are then reinstalled in the superconducting environment and a new reading is obtained to determine whether the desired balance condition has been attained. In actual practice, several such empirical attempts must bee made before the device is balanced, which is time consuming, relatively imprecise and thus undesirable.
In another known balancing scheme reported in the National Bureau of Standards Report No. 10,736 dated Mar. 31, 1972 entitled "Ultrasensitive Superconducting Magnetic Gradiometer", a plurality of superconducting vanes are movably arranged in the vicinity of the sensing coils in such a manner that the degree of ambient magnetic field coupling to one relative to the other is altered by shifting the position of the individual vanes. In this arrangement, transverse trimming is provided by vanes disposed about the periphery of the sensing coils while axial trimming is provided by shifting the axial position of a superconductive obstacle disposed intermediate and on the axis of a pair of sensing coils. This arrangement has been found to suffer from two disadvantages: viz., the adjustment afforded is highly non-linear; and an objectionable amount of mutual interaction exists between the various vane adjustments.
In still another arrangement, the inaccuracy and inconvenience in adjustment of the auxiliary trim coil type of balance adjustment mechanism note supra is reduced by having three trim coils, one for each orthogonal coordinate, and surrounding each trim coil with a movably positioned superconducting shield. By individually varying the position of each of the shields, the coupling of the ambient magnetic field to the associated trim coil is altered so that the sensing coils associated to the trim coils can be balanced. This arrangement suffers from the disadvantage that the trim coils must be carefully wound to precise specifications, which increases the overall cost of the device. Further, such coils have been found to be readily susceptible to damage during handling. In addition, the overall sensitivity of the sensing device is diminished by the introduction of the trim coils, and the trim coils further introduce problems of drift and noise.